Page 16 - Neutrons for Sciences and Society

Neutrons for Science
induced by these thermal neutrons. It follows that most nuclear reactors comprise a moderator of light material, with low neutron absorption (very pure graphite, light water, heavy water) arranged around and between bars of uranium. Fermi used graphite to construct the first reactor.
Neutrons have a wavelength as defined by the de Broglie equation relating to their velocity (hence energy):
λ = h/mv
Here h is Planck’s constant, m is the mass of the neutron and v its velocity. Fast neutrons have shortest wavelengths of the order of the dimensions of the nucleus. By contrast, thermal neutrons have wavelengths of the order of Angstroms i.e. comparable to inter-atomic distances. It is hence possible to observe diffraction effects with thermal neutrons in a similar way to X-rays. This
was understood in 1936 and the theory was published by W.M. Elsasser4. Later that year H. von Halban and P. Preiswerk showed the experimental proof5. This was immediately confirmed by D.P. Mitchell and P.N. Powers. These three articles are reproduced
in George Bacon’s book “Fifty Years of Neutron Diffraction” (1987)6. At the same time another discovery greatly increased interest in neutron diffraction. Felix Bloch, an American physicist, predicted7 that the neutron should possess a magnetic moment.
4 W.M. Elsasser, C.R. Acad. Sci., Paris, 202, 1029 (1936).
5 H. von Halban and P. Preiswerk, J. Phys. Radium, 8, 29-40 (1937), DOI 10.1051/jphysrad:019370080102900
6 “Fifty Years of Neutron Diffraction: The Advent of Neutron Scattering”, G.E. Bacon, Adam Hilger, Bristol
1987, ISBN 0-85274-587-7, DOI 10.1002/crat.2170221020
7 Felix Bloch, Phys. Rev., (1936) 50, 259, DOI 10.1103/PhysRev.50.259
7